1. Field of the Invention
The present invention relates to an optical communication system, and more particularly to a light source apparatus for optical communication using external-injection light sources. The present invention also relates to a method for improving transmission characteristics of the light source apparatus for optical communication and an optical communication system.
2. Description of the Related Art
Wavelength division multiplexed (WDM) passive optical networks have been highlighted as the next-generation optical network to provide future broadband communication services. Accordingly, efforts to economically implement such WDM passive optical networks are actively being made. WDM light sources are required for respective subscribers, since a particular wavelength is assigned to each subscriber in a WDM passive optical network.
In addition, a wavelength-division multiplexer/demultiplexer for optical signals generated from the WDM light sources is needed.
The most important factor for economical implementation of a WDM passive optical network is to reduce the cost of the WDM light sources and multiplexer/demultiplexer. For WDM light sources, use of a distributed feedback laser array, a light emitting diode, a spectrum-sliced source, etc. has been proposed.
Recently, an injection light source has also been proposed. The output wavelength of such a light source is not determined by the light source itself, but by light externally injected into the light source. This is done in order to achieve easy maintenance and repair of the light source. For such an injection light source, an injection Fabry-Perot laser diode (FP-LD) and a reflective semiconductor optical amplifier (R-SOA) have been proposed. Advantageously, injection light sources are usable for different wavelength channels without a particular adjustment. This is because the wavelength of each light source is determined by light injected therein.
Disadvantageously, the output light power of injection light sources is non-uniform for different wavelengths, thus transmission characteristics of optical signals are non-uniform.
Alternatively, a combination of arrayed waveguide gratings and thin film filters has been proposed for wavelength-division multiplexer/demultiplexers.
When arrayed waveguide gratings are used to implement a wavelength-division multiplexer/demultiplexer, they must have a wide free spectral range. This is needed in order to achieve a uniformity in insertion loss for wavelengths outputted from respective demultiplexing ports of the multiplexer/demultiplexer. However, utility of wavelengths is degraded.
FIG. 1 is a block schematic diagram illustrating a general thin film filter wavelength-division multiplexer/demultiplexer. As shown in FIG. 1, the thin film filter wavelength-division multiplexer/demultiplexer 10 includes n thin film filters (in the illustrated case, only four thin film filters are denoted by reference numerals 11 to 14, respectively) and waveguides 15, 16 and 17. Each thin film filter pass light of an associated wavelength and reflect light of wavelengths other than the associated wavelength.
Described first is the demultiplexing operation of the thin film filter wavelength-division multiplexer/demultiplexer 10. When broadband light is inputted to the waveguide 15, a first one of the thin film filters (i.e. the thin film filter 11) receives the broadband light from the waveguide 15. The light of a first wavelength to be outputted passes to an associated one of the waveguides 16. The first thin film filter 11 also reflects light of the remaining wavelengths to be inputted to a second one of the thin film filters (i.e. the thin film filter 12) via an associated one of the waveguides 17. The second thin film filter 12 passes light of a second wavelength to be outputted to an associated one of the waveguides 16. The second thin film filter 12 also reflects light of the remaining wavelengths to be inputted to a third one of the thin film filters (i.e. the thin film filter 13) via an associated one of the waveguides 17. This procedure is repeatedly carried out up to the n-th filter. In this manner the broadband light is demultiplexed into a plurality of wavelength components.
Described next is the multiplexing operation of the thin film filter wavelength-division multiplexer/demultiplexer 10. When light of particular wavelengths are applied to respective thin film filters (via waveguides 16), the light is multiplexed. The resultant multiplexed light is outputted via the waveguide 15.
Advantageously, such a thin film filter multiplexer/demultiplexer can be economically implemented. However, a non-uniformity in insertion loss occurs at different wavelengths due to different filter positions, thereby causing a non-uniformity in output light power. FIG. 2 is a graph depicting insertion loss distribution at different wavelengths according to positions of thin film filters in the thin film filter multiplexer/demultiplexer of FIG. 1. Referring to FIG. 2, it can be seen that a gradual increase in the insertion loss of input light occurs as the number of thin film filters, through which the input light passes, is increased. In this manner that light of the wavelength passing through the first thin film filter exhibits minimum insertion loss, whereas light of the wavelength passing through the last thin film filter exhibits maximum insertion loss. The power of injected light is reduced when the insertion loss increases. Although such a non-uniformity in insertion loss at different wavelengths may be eliminated (using an additional process) an increase in insertion loss is incurred in this case. Furthermore, there is a cost burden, thus, the advantages of the thin film filters compared to gratings disappear.